In the design of implantable defibrillation electrodes, it has traditionally been understood to be desirable to achieve a low impedance to the delivered current of the defibrillation pulse by employing low resistance materials, providing as large an electrode area as is practical for the implant site, and distributing that surface area over an even larger area. Early attempts to employ large diameter, endocardial leads bearing elongated, surface mounted, right ventricular electrodes showed that the necessary low impedance to allow use with implantable defibrillators was not possible to obtain. As a result, early automatic implantable cardioverter/defibrillators used large surface area, epicardial defibrillation patch electrodes that conformed to surface areas of the heart. However, the stress, pain and expense of the required thoracotomy to position the electrodes gave incentive to the development of the wide variety of endocardial defibrillation leads.
More recently, improvements in electrode materials and lead construction, the delivered shock waveforms and electrode combinations for optimal cardioversion pathways have allowed the clinical use of intracardiac electrodes in the right ventricle and/or right atrium.
One difficulty in achieving large surface area, distributed electrodes within the right ventricle (and the right atrium) lies in the physical constraints presented by the chamber itself. A further difficulty lies in the desire to make the lead body as small as possible to make the implantation easier. In addition, it is desired that the same lead carry the pace/sense unipolar or bipolar electrode(s) and a fixation mechanism to provide intimate contact of at least the distal pace/sense electrode with cardiac tissue. A further requirement is that there be no possibility whatsoever that the defibrillation electrode and the pace/sense electrode(s) contact one another. Finally, it is highly desirable or required that the lead be removable from the heart chamber after chronic use and consequent fibrosis of the lead.
Currently available implantable ventricular defibrillators typically employ epicardial or subcutaneous patch electrodes, alone, or in conjunction with one or more endocardial leads with one or more electrodes disposed within a heart chamber or blood vessel. Other contemplated multi-lead and multi-electrode atrial and/or ventricular defibrillation systems are widely disclosed, as exemplified in U.S. Pat. Nos. 4,708,145 to Tacker, et al., 4,998,975 to Cohen et al., 5,007,436 to Smits, 5,099,838 to Bardy, 5,107,834 to Ideker et al, 5,111,811 to Smits, 5,165,403 to Mehra, and 5,174,288 to Bardy et al.
Ventricular defibrillation is typically effected with at least one electrode disposed within the right ventricle extending along the length of the endocardial lead body and one or more electrodes disposed outside the right ventricle. Many versions of right ventricular defibrillation electrodes have been disclosed in the above listed patents and in further single endocardial lead systems as shown, for example, in further U.S. Pat. Nos. 4,481,953 to Gold et al., 4,161,952 to Kinney, et al., 4,934,049 to Kiekhafer et al., 5,010,894 to Edhag, 5,042,143 to Holleman, et al., 5,050,601 to Kupersmith et al., 5,133,365 to Heil, Jr. et al., and 5,144,960 to Mehra et al.
The positioning of at least a portion of the right ventricular (RV) lead electrode in proximity to the septum of the heart is considered to be desirable. In the above-referenced '975, '436, '811, '834 and '894 patents, U-shaped RV defibrillation leads are described and depicted which have at least a distal portion of the elongated defibrillation electrode shaped to bear against the septum. A U-shaped loop biased into the apex of the right ventricle is relied on to provide the force to press the distal portion back proximally along the septum. These RV leads either have a single or multiple defibrillation electrodes spaced along the lead body. In the '834 patent, the distal end of the lead bears a separate electrode that is intended to be directed into the outflow tract while more proximally located RV and RA/SVC electrodes are positioned in the right ventricle and the right atrium or superior vena cava, respectively.
In leads of this type, it is necessary to employ a separate endocardial lead having a distal pace/sense electrode that is wedged deep into the apex of the right ventricle or to rely on a ring shaped electrode as shown in the '834 patent. In the lead of the '834 patent, it is not possible to obtain the deep apical positioning of the pace/sense RV ring electrode, and pacing is compromised by the poor contact with myocardial cells. In the endocardial leads shown in the '894 and '975 patents, additional RV lead structures are disclosed that employ active or passive fixations mechanisms for fixing the RV pace sense electrodes on the lead bodies. A pair of RV defibrillation electrodes are formed on bifurcations of the lead body or in free legs that extend back from the point of attachment in the apex of the right ventricle. The disclosed lead system of the '975 patent otherwise includes epicardial electrodes positioned in a radical endocardial approach involving perforating the inferior vena cava wall.
The bifurcated and U-shaped lead bodies depicted in the '975 patent, for example, encounter a significant difficulty in effecting removal after chronic implantation. The fibrotic reaction encases the lead and electrode surfaces and grows through the bifurcation or inside the U-shape, rendering removal through traction very difficult if not impossible.
It is proposed in U.S. Pat. No. 5,257,634 to Kroll that the elongated electrode extending along the lead body be provided with relatively short conductor extensions that spring outward from the elongated electrode when the introducer catheter is withdrawn after placement of the electrode in the desired chamber. The conductor extensions only extend a short distance and effectively only increase the effective diameter of the electrode body and do not disperse the electrode surface over a wide area or volume.
In a further U.S. Pat. No. 5,143,089 to Alt, an epicardial lead is described having a distal electrode formed of a brush-like arrangement of fine, electrically conductive, carbonized polymer fibers to achieve high flexibility and distribution of the electrode surface area over a relatively large area. The electrode is introduced through the pericardium to a position adjacent the epicardium. The carbonized polymer fibers may break away. While this may not lead to any complications in the pericardial sac, the breakage and loss of such fibers could not be tolerated within the blood stream. The pace/sense electrode(s) are included on a RV endocardial lead so that the electrodes cannot short out on contact with the carbonized polymer fibers.
Despite these improvements, the achievement of appreciably lower cardioversion/defibrillation thresholds with current lead systems employing endocardial RV and RA electrodes continues to be a goal.